1. Field of the Invention
This invention relates to offshore apparatus for the production of oil and gas and more particularly to a structure for installing a riser at an offshore platform.
2. Description of the Prior Art
Most wells completed offshore have the wellhead structure supported on a platform. Production from the well is delivered to onshore facilities through a pipeline along the ocean bottom. The pipeline is connected to the flow line from the well through a riser that extends from the ocean bottom at the base of the platform to the deck of the platform, at which the wellhead is located. The riser usually has a bend of substantially 90.degree. for connection to the pipeline.
If the platform is in relatively shallow water, the connection between the lower end of the riser and the submerged pipeline is made by divers who weld the pipeline to the lower end of the riser. Many of the offshore wells now being completed are in water that is far deeper than the maximum depth at which divers can work. The connection of the pipeline to the riser at such deep water locations has been made by welding the pipeline to the bend of the riser at the platform deck or at the deck of a barge and gradually lowering the riser and pipeline to the bottom as additional lengths of pipe are welded to the riser, as disclosed in U.S. Pat. No. 3,698,199 to Matthews. This arrangement requires derrick barges which add greatly to the cost of making the connection. Moreover, the pipeline and riser are subjected to substantial strain as they are lowered to the ocean bottom.
U.S. Pat. No. 3,226,728 of Walvoord and U.S. Pat. No. 3,258,928 of Broadway et al disclose guides mounted on the framework of the derrick to support and guide the riser as it is lowered to the ocean bottom. In the structures described in both of the patents, the pipeline is subjected to a substantial bending force as it is lowered. U.S. Pat. Nos. 3,373,570 and 3,376,708, both of Hindman, disclose other guide structures for supporting the riser or the pipeline in which the final connection of the riser to the pipeline is made by a stabbing procedure.
In U.S. Pat. No. 3,690,112 of Pattison et al, the free end of the pipeline is connected to a pipe guide which is pulled into a structural guide at the base of the platform. A riser guide accurately positions the riser as it is lowered for stabbing connection to a vertical leg connected at the pipe guide to the end of the pipeline. U.S. Pat. No. 3,739,592 of Plake describes a platform structure in which the riser is encased within a casing. Locking cones secured to the casing are pulled into guide cones mounted on the platform to anchor the riser firmly in position. Presumably the pipeline is connected to the lower end of the riser by divers.